207
BARRETT PRC-4090 TACTICAL HF SDR TRANSCEIVER -APPENDICES
Appendix 3 - Overview of HF Operation
HF (High Frequency) is the radio spectrum with frequencies between 1.5 and
30 MHz. Within this radio spectrum an efficient form of transmitter modulation,
SSB (Single Side Band), is used. This, combined with the use of the ionosphere
- a layer of ionisation gases that resides between 100 and 700 km above the
Earth’s surface, provides efficient, cost effective communications over short,
medium and long distances - without the need for expensive re-transmission
devices, such as the VHF or UHF repeaters or satellites, all of which have ongo-
ing operational costs and a reliance on a physical infrastructure.
In many remote areas, HF / SSB is the only form of communication possible.
HF Propagation
When HF / SSB radio waves are generated by the transceiver there are usually
two components:
•
The ground-wave, which travels directly from the transmitting antenna to
the receiving antenna following the contours of the Earth.
•
The sky-wave, which travels upward and at an angle from the antenna,
until it reaches the ionosphere (an ionised layer high above the Earth’s sur-
face), and is then refracted back down to Earth, to the receiving antenna.
Generally speaking, ground-wave is used to communicate over shorter dis-
tances usually less than 50 km. Because ground-wave follows the contours of
the earth, it is affected by the type of terrain it passes over. Ground wave is rap-
idly reduced in level when it passes over heavily forested areas or mountainous
terrain.
Sky-wave is used to communicate reliably over medium to long distances up to
3,000 km. Whilst the nature of sky-wave propagation means it is not affected
by the type of terrain as in ground-waves, it is affected by factors involving the
ionosphere as described below.
